This application claims priority to an application entitled xe2x80x9cMethod of Fabricating Electro-Absorption Modulator Integrated Laserxe2x80x9d filed in the Korean Industrial Property Office on Jul. 4, 2000 and there duly assigned Serial No. 2000-37961.
1. Field of the Invention
The present invention relates generally to a laser diode for optical transmission, and in particular, to a method of fabricating an electro-absorption-modulator-integrated-laser (EML) having a laser diode and a modulator for an ultra high-speed optical communication network.
2. Description of the Related Art
In the field of transmitter devices for optical communications, the integration of a laser diode with a modulator in a buried heterostructure is used to provide a simple light transmission circuit at a low cost and less chirp. This device is used for transmitting digital signals through an optical fiber at a high speed in a wide range. Typically, an ultra high-speed optical transmission network requires a transmission speed of 2.5 gigabit per second (Gbps) or higher. To this end, a single-mode laser (e.g. a Distributed FeedBack laser) is used as a light source and an electro-absorption modulator is used as a switch. Signals resulting from applying a current to the active layer of the DFB (Distributed FeedBack) laser diode may be directly modulated to reach the higher frequency band. However, a direct modulation has a limited transmission distance because of chirp. To solve the transmission limit, external modulation using an electro-absorption (EA) light modulator may be employed for long distance fiber optic communication over several tens to several hundreds of kilometers or more at a high speed of 2.5-10 gigabit per second (Gb/s) or higher. One way to achieve this type of high-speed transmission is utilizing an electro-absorption-modulator-integrated-laser (EML). The use of EML has advantages in obtaining relatively small optical loss during modulation. The EML also allows a compact module due to the integrated structure and requires a low driving voltage.
Since a semiconductor laser part is a forward bias device and a modulator part is a reverse bias device, the characteristics of an EML are determined by isolation between these two devices. As shown in FIG. 1, the conventional EML is fabricated by electrically isolating the laser diode region 11 from a modulator region 12 through either the isolation etching or ion implantation process. In the isolation etching, an isolation region is formed between the DFB laser diode and the EA modulator by etching away both the cap and the middle of cladding. Here, the resistance between two devices varies according to the amount of etching. Thus, the isolation etching process should be controlled accurately. In the ion implantation process, the electrical isolation can be achieved without etching away the cap and cladding layers, by using deep ion implantation in the region between the laser diode and the modulator. However, it takes a long time to find out an optimal condition for the ion implantation, and it is also difficult to maintain the optimal condition due to adverse effect by the ion implantation on the grown crystals.
It is, therefore, an object of the present invention to provide an EML fabricating method that obviates the need for a separate isolation etching control requirement and the ion implantation process.
To achieve the above object, an EML is fabricated by the following steps: preparing a compound semiconductor structure in which a laser diode is directly integrated with a modulator simultaneously; forming a two step InP layer, comprised of p-Inp and undoped InP layers, on the compound semiconductor structure, forming an InGaAs layer on the undoped InP layer; forming a mask layer defining a trench region between the laser diode and the modulator regions; depositing Zn or a Zn compound in a metal contact forming area on the laser diode and the modulator regions, except for the trench area; diffusing the Zn; and, selectively etching the mask layer and the InGaAs layer to a predetermined depth.